Chromium/Titanium/Aluminum-based Semiconductor Device Contact

ABSTRACT

A contact to a semiconductor including sequential layers of Cr, Ti, and Al is provided, which can result in a contact with one or more advantages over Ti/Al-based and Cr/Al-based contacts. For example, the contact can: reduce a contact resistance; provide an improved surface morphology; provide a better contact linearity; and/or require a lower annealing temperature, as compared to the prior art Ti/Al-based contacts.

REFERENCE TO PRIOR APPLICATIONS

The current application is a continuation-in-part of U.S. applicationSer. No. 12/102,408, titled “Chromium/Titanium/Aluminum-basedSemiconductor Device Contact,” which was filed on 14 Apr. 2008, andwhich claims the benefit of U.S. Provisional Application No. 60/937,054,titled “Contact for semiconductor devices”, which was filed on 25 Jun.2007, each of which is hereby incorporated by reference.

TECHNICAL FIELD

The disclosure relates generally to device contacts, and moreparticularly, to contacts for a semiconductor device, such as a GalliumNitride semiconductor device.

BACKGROUND ART

The formation of a contact for a semiconductor device is an importantprocess in fabricating the semiconductor device. For example, contactresistance significantly affects the performance characteristics of thesemiconductor device. As a result, achieving a low contact resistance istypically desired. Other important characteristics include contactstability, surface morphology, reliability, and linearity, especially atlarge currents.

In order to achieve a low contact resistance for Gallium Nitride (GaN)devices, several contact metals and a relatively high annealingtemperature are typically utilized to form the contact. Aluminum (Al) iswidely used in ohmic contacts because of its low melting point of 660degrees Celsius. Additionally, Titanium (Ti) or Chromium (Cr) is used asthe first layer for the contact because of its low metal work functionto GaN. For example, one approach forms a Ti/Al-based contact to ann-type GaN semiconductor having several metals, such as Ti/Al/Ti/Gold(Au) or Ti/Al/Nickel (Ni)/Au, a thickness from five nanometers to fivemicrons, and which is annealed at 400 degrees Celsius or more. Adifferent approach reverses the order of Ti and Al, and forms anAl/Ti-based contact to an n-type GaN semiconductor that includesAl/Ti/Platinum (Pt)/Au and is annealed at temperatures between 400 and600 degrees Celsius. Other approaches form a Cr/Al-based contact to ann-type GaN semiconductor that include various metal configurations, suchas Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Lead (Pd)/Au, Cr/Al/Ti/Au,Cr/Al/Cobalt (Co)/Au, and Cr/Al/Ni/Au.

To date, a Ti/Al-based contact yields a lower contact resistance than aCr/Al-based contact, while requiring a higher temperature annealing forthe contact alloy. However, the Ti/Al-based n-contact is not reliablefor 265 nanometer (nm) and shorter wavelength ultraviolet (UV) lightemitting diodes (LEDs). The Cr/Al-based contact has a lower annealingtemperature, but a higher contact resistance, than the Ti/Al-basedcontact. Because of the lower annealing temperature, the surfacemorphology of the Cr/Al-based contact is better than that of theTi/Al-based contact.

Various research has been devoted to improving the Ti/Al-based contactsand the Cr/Al-based contacts.

SUMMARY OF THE INVENTION

Aspects of the invention provide a novel Cr/Ti/Al-based contact to asemiconductor, which includes sequential layers of Cr, Ti, and Al. In anembodiment, a contact including a Cr/Ti/Al alloy is provided, which canresult in a contact that provides numerous advantages, including atleast some of those of the prior art Ti/Al-based and Cr/Al-basedcontacts. The resulting contact can have one or more advantages overTi/Al-based and Cr/Al-based contacts. For example, the contact can:reduce a contact resistance; provide an improved surface morphology;provide a better contact linearity; and/or require a lower annealingtemperature, as compared to the prior art Ti/Al-based contacts.

A first aspect of the invention provides a contact to a semiconductorcomprising: Chromium; Titanium directly on the Chromium; and Aluminumdirectly on the Titanium.

A second aspect of the invention provides a semiconductor devicecomprising: a semiconductor structure having an exposed surface and asemiconductor below the exposed surface; and a contact to thesemiconductor, the contact including: Chromium over the exposed surface;Titanium directly on the Chromium; and Aluminum directly on theTitanium.

A third aspect of the invention provides a method of fabricating asemiconductor device, the method comprising: obtaining a semiconductorstructure having an exposed surface and a semiconductor below theexposed surface; and forming a contact to the semiconductor, the contactincluding: Chromium over the exposed surface; Titanium directly on theChromium; and Aluminum directly on the Titanium.

A fourth aspect of the invention provides an integrated circuitcomprising: a first semiconductor device including: a semiconductorstructure having an exposed surface and a semiconductor below theexposed surface; and a first contact to the semiconductor, the firstcontact including: Chromium over the exposed surface; Titanium directlyon the Chromium; and Aluminum directly on the Titanium; and a seconddevice having a second contact, wherein the first contact provides aninterconnect to the second contact.

Other aspects of the invention provide methods, systems, and methods ofusing and generating each, which include and/or implement some or all ofthe actions described herein. The illustrative aspects of the inventionare designed to solve one or more of the problems herein describedand/or one or more other problems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIGS. 1A-B show illustrative contacts according to embodiments.

FIG. 2 shows illustrative n-contact transmission line method (TLM)measurement results of a 255 nm UV LED having a width (W) of 200 μmaccording to an embodiment.

FIG. 3 shows illustrative n-contact TLM measurement results of a 265 nmUV LED having a width (W) of 200 μm according to an embodiment.

FIG. 4 shows illustrative n-contact edge and center TLM current-voltage(IV) characteristic measurements for a 255 nm UV LED having awidth/length (W/L) of 200 μm/4 μm according to an embodiment.

FIG. 5 shows illustrative n-contact TLM measurement results of a 280 nmUV LED having a width (W) of 200 μm and different Cr thicknessesaccording to embodiments.

FIGS. 6A-B show illustrative CCD pictures of Ti/Al-based contacts andCr/Ti/Al-based contacts, respectively, for a 280 nm UV LED according toan embodiment.

FIG. 7 shows illustrative n-contact TLM measurement results for a fieldeffect transistor having a width (W) of 200 μm and different Crthicknesses according to embodiments.

FIGS. 8A-B show illustrative charge-coupled device (CCD) pictures ofTi/Al-based contacts and Cr/Ti/Al-based contacts, respectively, for aheterostructure field effect transistor (HFET) according to anembodiment.

It is noted that the drawings are not to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a novelCr/Ti/Al-based contact to a semiconductor, which includes sequentiallayers of Cr, Ti, and Al. In an embodiment, a contact including aCr/Ti/Al alloy is provided, which can result in a contact with thecombined advantages of the prior art Ti/Al-based and Cr/Al-basedcontacts. The resulting contact can have one or more advantages overTi/Al-based and Cr/Al-based contacts. For example, the contact can:reduce a contact resistance; provide an improved surface morphology;provide a better contact linearity; and/or require a lower annealingtemperature, as compared to the prior art Ti/Al-based contacts. As usedherein, unless otherwise noted, the phrase “any solution” means any nowknown or later developed solution.

Turning to the drawings, FIGS. 1A-B show illustrative contacts 10A-B,respectively, according to embodiments. As illustrated, contacts 10A-Bare formed on an exposed surface of a semiconductor structure 12.Semiconductor structure 12 can comprise at least one layer thatcomprises a semiconductor 12A. For example, semiconductor structure 12can comprise one or more layers/regions of: a binary, ternary, orquaternary group III-V material (e.g., AlN, GaN, InN, and/or the like)or alloy(s) thereof; diamond; any carbon modification; silicon carbide(including all polytypes or mixtures thereof); silicon; germanium; asilicon germanium alloy; and/or the like. Further, semiconductorstructure 12 may include one or more layers/regions of insulatingmaterial, metals, and/or the like. The precise configuration oflayers/regions can be selected to obtain desired operatingcharacteristic(s) of a resulting semiconductor device using anysolution. In an embodiment, semiconductor 12A comprises an n-typesemiconductor material. In a further embodiment, the n-typesemiconductor material is a GaN-based semiconductor material, such asAlGaN, an AlGaN/GaN heterostructure, or the like. In a still furtherembodiment, semiconductor 12A can comprise a layer, such as an AlGaNlayer, having a high Al composition.

Each contact 10A-B is a contact to semiconductor 12A, which is disposedbelow contact 10A-B. Contact 10A or 10B can be formed directly on anexposed surface of semiconductor 12A as illustrated by contact 10A inFIG. 1A. Alternatively, contact 10A or 10B can be formed on an exposedsurface of a layer that is disposed between semiconductor 12A andcontact 10A or 10B. To this extent, as illustrated by contact 10B inFIG. 1B, contact 10A or 10B can comprise a contact to a metal-insulatorsemiconductor structure. In particular, contact 10A or 10B can be formedon an insulating layer 12B, which is disposed between semiconductor 12Aand contact 10B. Insulating layer 12B can comprise any type ofinsulating material, such as silicon dioxide, silicon nitride, aluminumoxide, aluminum nitride, and/or the like. Alternatively, another type oflayer can separate contact 10A or 10B from semiconductor 12A, such as athin semiconductor layer or the like. An illustrative thin semiconductorlayer can comprise a thin layer of aluminum nitride, silicon, or thelike.

In any event, contacts 10A-B can be formed on an exposed surface of alayer (semiconductor, insulating, or the like) of semiconductorstructure 12 using any solution. For example, semiconductor structure 12having the exposed surface can be obtained using any solution (e.g.,manufactured, purchased, prepared, and/or the like). Subsequently,contact 10A can be formed on the exposed surface using any solution. Forexample, Chromium 14 can be deposited directly on the exposed surfaceusing any solution, Titanium 16 can be deposited directly on Chromium14, and Aluminum 18 can be deposited directly on Titanium 16.Additionally, a barrier layer 20 can be formed (e.g., deposited)directly on Aluminum 18, and Gold 22 can be deposited directly onbarrier layer 20. Barrier layer 20 can comprise any metal, such asTitanium, Chromium, Nickel, Cobalt, Platinum, Lead, or the like. Thethickness of each metal in contact 10A can vary between approximately 10Angstroms and approximately 1 micron.

Similarly, contact 10B can be formed on an exposed surface ofsemiconductor structure 12 using any solution. For example, aninterfacial layer 24 can be formed (e.g., deposited) directly on theexposed surface using any solution, Interfacial layer 24 can provide aninsulating layer on which contact 10B is formed. Interfacial layer 24can comprise a semi-metal, a semiconductor having an energy gap that issmaller than an energy gap of the material to which contact 10B is made(e.g., semiconductor 12A), an amorphous material, a doped (e.g., highlydoped) semiconductor, a semiconductor superlattice, and/or the like. Inan embodiment, interfacial layer 24 comprises aluminum gallium nitride.Interfacial layer 24 can have a thickness between approximately 10Angstroms and approximately 1 micron. The remainder of contact 10B canbe formed on interfacial layer 24 in the same manner as described forcontact 10A.

Either contact 10A-B can comprise an ohmic contact or a Schottkycontact. Additionally, either contact 10A-B can comprise a contact to ap-type or n-type semiconductor 12A. Further, the formation of contact10A-B can include annealing the contact 10A-B to improve the contactperformance. In an embodiment, the annealing is performed at atemperature between approximately 500 degrees Celsius and approximately1100 degrees Celsius, and will vary based on the composition ofsemiconductor structure 12, among other factors.

Contact 10A-B and semiconductor structure 12 can be implemented as partof various types of semiconductor devices using any solution. To thisextent, contact 10A-B can be used in wide band gap semiconductortechnology and semiconductor devices, such as those used in microwaveamplifiers, oscillators, switchers, and/or the like, as well asoptoelectronic devices. For example, illustrative types of devicesinclude a light emitting diode (LED), a laser, a field effect transistor(FET), a solar cell, a charge coupled device, a Schottky diode, a p-njunction diode, and/or the like. Further, a device fabricated withcontact 10A-B can be implemented in an integrated circuit, in whichcontact 10A-B provides an interconnect to a contact for another devicein the circuit.

The contact described herein has been implemented on several devicesusing several alternative embodiments and evaluated with respect toprior art Ti/Al-based contacts. The evaluation has shown thatembodiments of the contact can: reduce a contact resistance; provide animproved surface morphology; provide a better contact linearity; and/orrequire a lower annealing temperature, as compared to the prior artTi/Al-based contacts.

For example, when implemented as a contact to an AlGaN semiconductorwith a relatively high molar fraction of Al (e.g., a molar fraction ofAl that is more than 50%), an embodiment of the contact described hereincan reduce a contact resistance and/or provide a more uniform contactresistance than a Ti/Al-based contact. The AlGaN semiconductor with arelatively high molar fraction of Al is commonly used for deepUltraviolet (UV) LEDs that emit electromagnetic radiation having awavelength between approximately 255 nm and 265 nm.

To this extent, FIG. 2 shows illustrative n-contact TLM measurementresults of a 255 nm UV LED having a width (W) of 200 μm according to anembodiment. As illustrated, the n-contact resistivity was approximately3.6 Ω-mm for the Cr/Ti/Al-based contact, and approximately 23.4 Ω-mm forthe Ti/Al-based contact. To this extent, the Cr/Ti/Al-based contactreduced the contact resistance by approximately 84.6%. FIG. 3 showsillustrative n-contact TLM measurement results of a 265 nm UV LED havinga width (W) of 200 μm according to an embodiment. In this case, then-contact resistivity was approximately 1.3 Ω-mm for the Cr/Ti/Al-basedcontact and approximately 1.5 Ω-mm for the Ti/Al-based contact, whichresults in an approximately 13.3% reduction in contact resistance.Additionally, FIG. 4 shows illustrative n-contact edge and center TLM IVcharacteristic measurements for a 255 nm UV LED having a width/length(W/L) of 200 μm/4 μm according to an embodiment. As illustrated, theresistance of an embodiment of the Cr/Ti/Al-based contact is moreuniform than that of the Ti/Al-based contact.

For UV LEDs that emit radiation having a wavelength betweenapproximately 265 nm and 295 nm, the contact resistance for anembodiment of Cr/Ti/Al-based contact was similar to the contactresistance for Ti/Al-based contacts. For example, as shown in FIG. 3,the Cr/Ti/Al-based contact provided a slight improvement in contactresistance for a Ti/Al-based contact for a 265 nm UV LED. FIG. 5 showsillustrative n-contact TLM measurement results of a 280 nm UV LED havinga width (W) of 200 μm and different Cr thicknesses according toembodiments. As illustrated, the contact resistivity is approximately:1.7 Ω-mm for a Cr thickness of 5 nm; 1.2 for a Cr thickness of 10 nm;and 0.9 for a Cr thickness of 20 nm. While the contact resistivityimproved with the Cr thickness, the 20 nm Cr thickness requires a longerannealing time.

The annealing temperature for embodiments of the Cr/Ti/Al-based contactare also similar to that of Ti/Al-based n-contacts for 265 nm to 295 nmUV LEDs. However, embodiments of the Cr/Ti/Al-based contact showed animproved surface morphology over that of a Ti/Al-based contact. To thisextent, FIGS. 6A-B show illustrative CCD pictures of Ti/Al-basedcontacts and Cr/Ti/Al-based contacts, respectively, for a 280 nm UV LEDaccording to an embodiment. As illustrated, the image of theCr/Ti/Al-based contacts (FIG. 6B) is a lighter color than the image ofthe Ti/Al-based contacts (FIG. 6A). The lighter color results from morereflected light from a more uniform surface. This is an indication ofimproved morphology of the Cr/Ti/Al-based contacts over that of theTi/Al-based contacts.

For n-GaN-based blue LEDs, the annealing temperature is between 400degrees Celsius to 600 degrees Celsius for Ti/Al-based contacts.However, a higher Al composition in UV LEDs requires a higher annealingtemperature for forming an ohmic contact. For example, for n-type AlGaNUV LEDs that emit radiation having a wavelength between approximately330 nm and 365 nm, a Ti/Al-based contact requires an annealingtemperature of approximately 850 degrees. Embodiments of theCr/Ti/Al-based n-contacts for 330 nm to 365 nm UV LEDs require a lowerannealing temperature, e.g., approximately 700 degrees Celsius. Forlow-Al concentration AlGaN materials, such as those used in green LEDs,blue LEDs, and in field effect transistors, both the Ti/Al-based andCr/Ti/Al-based contacts give satisfactory results.

Table 1 below compares the annealing temperatures for Ti/Al andCr/Ti/Al-based contacts to AlGaN with different molar fractions ofAluminum (e.g., in UV LEDs generating radiation of differentwavelengths). Similarly, Table 2 compares the contact resistances ofTi/Al and Cr/Ti/Al-based contacts to AlGaN with different molarfractions of Aluminum. As illustrated, Cr/Ti/Al-based contacts havelower resistances compared to Ti/Al-based contacts for deep UV LEDs(255-265 nm). The advantage is more pronounced for shorter wavelengths.Additionally, the Cr/Ti/Al-based contacts have lower annealingtemperatures compared to Ti/Al-based contacts for longer wavelength UVLEDs (330-365 nm). Still further, the current-voltage (IV)characteristics of Cr/Ti/Al yield a low contact resistance for allresearched UV LEDs (e.g., 255-365 nm).

TABLE 1 n-contact anneal temperatures Wavelength 365 340 295 280 265 255nm nm nm nm nm nm Ti/Al- 850° C. 850° C. 900° C. 900° C. 900° C. 950° C.contact Cr/Ti/Al- 700° C. 700° C. 900° C. 900° C. 900° C. 950° C.contact

TABLE 2 n-contact resistance Wavelength 365 340 295 280 265 255 nm nm nmnm nm nm Ti/Al-contact (Ω · mm) 0.18 0.24 0.56 1.00 1.5 23.4Cr/Ti/Al-contact (Ω · mm) 0.75 0.49 0.78 1.23 1.3 3.6

As previously described, the Cr/Ti/Al-based contact can be implementedon various types of devices in addition to UV LEDs. For example,embodiments of the Cr/Ti/Al-based contact have been used for anAlGaN/GaN HFET n-contact process. To this extent, FIG. 7 showsillustrative n-contact TLM measurement results for a field effecttransistor having a width (W) of 200 μm and different Cr thicknessesaccording to embodiments. As illustrated, the contact resistivity isapproximately: 0.5 Ω-mm for a Cr thickness of 5 nm; 0.36 for a Crthickness of 10 nm; and 0.4 for a Cr thickness of 20 nm. While thecontact resistance could be smaller than that of the Ti/Al-basedcontact, any difference is not large.

However, the Cr/Ti/Al-based contact can provide an improved alloymorphology over that of Ti/Al contacts due to the lower annealingtemperature. For example, FIGS. 8A-B show illustrative CCD pictures ofTi/Al-based contacts and Cr/Ti/Al-based contacts, respectively, for anHFET according to an embodiment. As illustrated, the image of theCr/Ti/Al-based contacts (FIG. 8B) is a lighter color than the image ofthe Ti/Al-based contacts (FIG. 8A). The lighter color results from morereflected light from a more uniform surface. This is an indication ofimproved morphology of the Cr/Ti/Al-based contacts over that of theTi/Al-based contacts.

Further, as previously discussed with reference to FIG. 4, the contactresistance (e.g., voltage divided by current) for Cr/Ti/Al-based contactis more uniform from edge to center than that of the Ti/Al-basedcontact. As a result of the better contact quality, a device, such as anHFET or a UV LED, including Cr/Ti/Al-based contact(s) may have animproved thermal stability for the contact(s) and an improvedoperational lifetime as compared to that provided by the Ti/Al-basedcontact(s).

Returning to FIGS. 1A and 1B, in an embodiment, fabrication of aCr/Ti/Al-based contact 10A, 10B as described herein can result in adiscontinuous Chromium layer 14. In this case, portions of theunderlying material, such as the semiconductor 12A and/or interfaciallayer 24, will directly contact the Titanium layer 16, while otherportions will include the intervening Chromium layer 14. For example,the Chromium layer 14 can be deposited and/or annealed duringfabrication of the contact 10A, 10B using a solution, which results information of disconnected islands of Chromium on the underlying materialand/or disconnected islands of the underlying material extending abovethe Chromium layer 14.

In an illustrative embodiment, a surface roughness of the semiconductorlayer 12A and/or the interfacial layer 24 is in a range betweenapproximately twenty nm and approximately thirty nm. A total thicknessof each of the Chromium layer 14 and the Titanium layer 16 can be in arange between approximately ten and approximately twenty nm, with atotal thickness of the Chromium layer 14 and Titanium layer 16 beingclose to the surface roughness of the semiconductor layer 12A and/or theinterfacial layer 24. When formed using, for example, e-beam deposition,the relatively thin Chromium layer 14 and Titanium layer 16 will notcover the corresponding surface of the semiconductor layer 12A or theinterfacial layer 24 uniformly. Subsequently, during annealing, such asa high temperature annealing higher than approximately 850° C., Aluminumin the Aluminum layer 18 will alloy with the Titanium and Chromium,forming TiAl and CrAl alloys, which result in a discontinuous Chromiumlayer 14.

As described herein, the semiconductor 12A can comprise a group IIInitride material having a high Aluminum content. For example, thesemiconductor 12A can comprise an Al_(x)Ga_(1-x)N layer of materialwhere x is greater than approximately 0.3. In a more particularembodiment, such a layer of material can be included in a deepultraviolet LED. Illustrative semiconductors 12A include: an AlGaN layerhaving an aluminum content of approximately 0.3 for a 360 nanometer LEDand an AlGaN layer having an aluminum content of approximately 0.5 for a280 nanometer LED, and an AlGaN layer having an aluminum content ofapproximately 0.6 for a 260 nanometer LED. For shorter wavelength groupIII nitride-based UV LEDs, the aluminum content is increased from 0.3 to0.8 to obtain a higher energy band-gap of the materials, which moves theemission wavelength in a range from approximately 360 nm toapproximately 230 nm. However, the higher band-gap of the materialsincreases a work function between a metal for an n-type contact and thesemiconductor 12A. The increased work function can cause a poor n-typeohmic contact, and can result in a higher contact resistance. Forexample, a higher aluminum content in the semiconductor 12A can makeformation of a TiN alloy to reduce the metal work function for an ohmiccontact more difficult. To this extent, an annealing temperature can beincreased to overcome problem(s) presented by a higher aluminum contentin the semiconductor 12A.

In an embodiment, the underlying material, such as the semiconductor 12Aand/or the interfacial layer 24, can be configured to facilitateformation of the discontinuous Chromium layer 14. For example, prior toforming the contact 10A, 10B, a surface of the semiconductor 12A and/orthe interfacial layer 24 can be prepared for the contact formation. Inan embodiment, the preparation includes roughening or patterning anexposed surface of the semiconductor 12A and/or the interfacial layer 24(e.g., using etching or the like) prior to depositing the Chromium layer14 thereon. In this case, a thickness of the Chromium layer 14 can beselected to allow at least a portion of the roughened surface to extendthere through.

A discontinuous Chromium layer 14 can, for example, result in a contact10A, 10B having improved optical properties over that provided by acontact 10A, 10B with a continuous Chromium layer 14. The improvedoptical properties can include: an increased light scattering, anincrease in reflectivity due to a larger portion of the light beingreflected by the more reflective contact layers, such as the Aluminumlayer 18, and/or the like. In an embodiment, the metallic contact 10Bincludes a discontinuous Chromium layer 14 and the interfacial layer 24is configured to promote current spreading in discontinuous portion(s)of the Chromium layer. For example, the interfacial layer 24 can haveresistive properties such that, during operation of the correspondingdevice, a sufficient amount of current spreading is present in thediscontinuous portion(s) of the Chromium layer 14 through theinterfacial layer 24. In an illustrative embodiment, the interfaciallayer 24 can be configured to provide a contact resistance similar to acontact resistance of a contact having a continuous Chromium layer withno interfacial layer 24. As used herein, the two resistances are similarwhen one resistance is no more than approximately twenty percent higherthan the other resistance.

In an embodiment, the interfacial layer 24 can be formed using anovergrowth procedure. For example, at least a portion of an exposedsurface of the semiconductor layer 12A can be etched using any solution.Subsequently, the interfacial layer 24 (and/or the insulating layer 12B)can be grown over the etched portion of the semiconductor layer 12A. Itis understood, that when present, the insulating layer 12B can bediscontinuous to allow current to flow from the contact layer 24 intothe semiconductor layer 12A. Such an insulating layer 12B can comprise,for example, a patterned layer including insulating domains forming aset of islands. Alternatively, a patterned insulating layer can comprisea layer of material with openings formed therein.

In an illustrative embodiment, an overgrowth procedure described in U.S.patent application Ser. No. 13/775,038, which is hereby incorporated byreference, is used to form the interfacial layer 24 and the ohmiccontact 10B, which can be performed without etching the semiconductorlayer 12A. For example, the semiconductor layer 12A can be patterned bya process including: depositing a masking material; overgrowing unmaskedregions with the interfacial layer 24; and removing the maskingmaterial. The masking material can comprise any suitable material, suchas SiO₂, Si₃N₄, and/or the like.

While primarily shown and described as a contact and a semiconductordevice, it is understood that the invention provides a method offabricating such a contact and/or semiconductor device. For example, asshown in FIGS. 1A-B, the semiconductor device can be fabricated byobtaining a semiconductor structure 12 having an exposed surface and asemiconductor 12A below the exposed surface and forming contact 10A or10B to semiconductor 12A on the exposed surface. As described herein,contact 10A or 10B includes Chromium over the exposed surface, Titaniumdirectly on the Chromium, and Aluminum directly on the Titanium. Eithercontact 10A-B can include one or more additional metals/layers, whichcan be formed using any solution. Further, the formation of contact 10Aor 10B can include annealing the contact, e.g., at a temperature betweenapproximately 500 degrees Celsius and approximately 1100 degreesCelsius. Formation of the semiconductor device can include variousadditional known steps/processes, which are not described herein forclarity.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A method comprising: forming an ohmic contact ona semiconductor structure, wherein the forming includes: forming amulti-layer structure comprising: the semiconductor structure; aChromium layer over the semiconductor structure; a Titanium layerdirectly on the Chromium layer; and an Aluminum layer directly on theTitanium layer; and annealing the multi-layer structure.
 2. The methodof claim 1, the multi-layer structure further comprising a barrier layerdirectly on the Aluminum layer.
 3. The method of claim 2, the barrierlayer comprising at least one of: Titanium, Chromium, Nickel, Cobalt,Platinum, or Lead.
 4. The method of claim 2, the multi-layer structurefurther comprising a Gold layer directly on the barrier layer.
 5. Themethod of claim 1, wherein the Chromium layer is discontinuous.
 6. Themethod of claim 5, further comprising preparing a surface of thesemiconductor structure prior to forming the Chromium layer, wherein thepreparing includes roughening a surface of the semiconductor structure.7. The method of claim 5, the multi-layer structure further comprisingan interfacial layer on the semiconductor structure, wherein theChromium layer is directly on the interfacial layer, and wherein theinterfacial layer is configured to promote current spreading in a set ofdiscontinuous portions of the Chromium layer.
 8. The method of claim 7,wherein the method further includes forming the interfacial layer usingan overgrowth procedure, and wherein the overgrowth procedure isperformed without etching the semiconductor structure.
 9. The method ofclaim 1, wherein the ohmic contact is to a group III nitride layer inthe semiconductor structure, and wherein the group III nitride layer hasan Aluminum content of at least approximately 0.3.
 10. A method offabricating a semiconductor device comprising: forming an ohmic contacton a semiconductor structure of the semiconductor device, wherein theforming includes: forming a multi-layer structure comprising: thesemiconductor structure; a Chromium layer over the semiconductorstructure; a Titanium layer directly on the Chromium layer; and anAluminum layer directly on the Titanium layer; and annealing themulti-layer structure.
 11. The method of claim 10, wherein the Chromiumlayer is discontinuous.
 12. The method of claim 11, further comprisingpreparing a surface of the semiconductor structure prior to forming theChromium layer, wherein the preparing includes roughening a surface ofthe semiconductor structure.
 13. The method of claim 11, the multi-layerstructure further comprising an interfacial layer on the semiconductorstructure, wherein the Chromium layer is directly on the interfaciallayer, and wherein the interfacial layer is configured to promotecurrent spreading in a set of discontinuous portions of the Chromiumlayer.
 14. The method of claim 13, wherein the method further includesforming the interfacial layer using an overgrowth procedure, and whereinthe overgrowth procedure is performed without etching the semiconductorstructure.
 15. The method of claim 10, wherein the ohmic contact is to agroup III nitride layer in the semiconductor structure, and wherein thegroup III nitride layer has an Aluminum content of at leastapproximately 0.3.
 16. The method of claim 10, wherein the device is oneof: a light emitting diode, a laser, a field effect transistor, a solarcell, a charge coupled device, a Schottky diode, or a p-n junctiondiode.
 17. A method of fabricating a light emitting device, the methodcomprising: forming an ohmic contact on a semiconductor structure of thelight emitting device, wherein the forming includes: forming amulti-layer structure comprising: the semiconductor structure; aChromium layer over the semiconductor structure; a Titanium layerdirectly on the Chromium layer; and an Aluminum layer directly on theTitanium layer; and annealing the multi-layer structure.
 18. The methodof claim 17, wherein the annealing is performed at a temperature betweenapproximately 500 degrees Celsius and approximately 1100 degreesCelsius.
 19. The method of claim 17, wherein the Chromium layer isdiscontinuous.
 20. The method of claim 17, wherein the ohmic contact isto a group III nitride layer in the semiconductor structure, and whereinthe group III nitride layer has an Aluminum content of at leastapproximately 0.3.